In a collectorless DC motor, the electronic commutation arrangement must always switch-on the coils when they have a favorable position with respect to the rotating field magnet for torque generation.
The positional recognition of the rotating rotor is generally determined by means of Hall probes. These supply a pulse voltage under the influence of the magnetic field of the rotor, which voltage is utilized for controlling the electronic commutation arrangement. The quantity and arrangement of the Hall probes must be selected corresponding to the number of phases of the stator winding. Hall integrated circuits, which are also utilized instead of normal Hall probes, also comprise integrated electronics.
A number of disadvantages are involved in this up-to-now extensively used technology:
(1) Semiconductor components, to which Hall probes and Hall integrated circuits belong, have a negative temperature coefficient. As a result, they exhibit a temperature dependence of their resistances and with it also of their output signals. For this reason, problems can occur in the course of signal recognition. The level of the signals can within certain limits be compensated by the electronic circuits. This requires an added expenditure.
(2) The temperature sensitivity of the semiconductor components causes a high failure rate.
(3) The utilization area of the Hall probes and Hall integrated circuits is further limited by their sensitivity to radiation. In a radiation intensive environment, for instance, in accelerator installations, defects in semiconductor components increase, which leads to improper functioning.
(4) A further disadvantage is the number of connector lines from the Hall probes to the electronic commutation arrangements. Six lines are required for each Hall probe, which results in an undesirable expense when several probes are utilized. The length of the connector lines is limited, since phase errors of the signals are caused by the capacitances of the cables and, additionally, inducement of malfunctions in the current supply to the motor are unavoidable. The useful signal-to-noise ratio is then very low because of the low level of the useful signal.